Modulation system



July 30,1940.

N. Rus-r MODULATION SYSTEM Filed April 18, 1936 IIIIIIII DETEC Toe INVENTOR NOEL 'MEYEIQ U57 BY 8 ATTORNEY OSC'IL LATOE Patented July 30,1940 I PATENT OFFICE 2,209.54; MODULATION SYSTEM Noel Meyer Rust, Chelmsiord, England, assignor to Radio Corporation of America, a corporation of Delaware Application April 18, 1936, Serial No..75,056

, In Great Britain May 11, 1935 4 Claim.

This invention relates to microphones and circuit arrangements incorporating the same.

It has been proposed in microphone arrangements to eliminate bafl'ie and similar effects due 5 to the size of the microphone relative to the wave length of the sound to be picked up thereby araranging the microphone-in association with an acoustic conduit of small physical dimensions relative to the wave lengths of the higher frequency sounds. In this way, the pickup oi the microphone is caused to be, to all intents and purposes, the same as or very nearly similar to the pickup which would be obtained if it were actuated by sound waves in free space without any baflle effects.

The present invention may be regarded as providing an electrical arrangement somewhat analogous to the acoustic arrangement just described, and according to this invention a microphone or. microphone member is utilized as an acoustically variable impedance in an electrical line of a predetermined length in relation to electrical energy which is fed thereto, so that acoustically produced relatively small changes in the impedance of the microphone or microphone member may be caused to produce relatively large changes in the degree of modulation imparted to the energy fed to the line and microphone or microphone member combination. The invention is illustrated in the accompanying schematic drawing wherein Figures 1 and 2 illustrate a velocity microphone of the magnetophone type for use in my improved modulation system. Figures 3 and 4, illustrate a pressure microphones of the type illustrated in Figures 1,

2, 3 and 4. 40 In one way of carrying out this invention, a so-called ribbon armature microphone of the magneto-phone type is mounted without the normally provided magnet system at the end of an electrical line a quarter of a wave length long (at a predetermined frequency) the microphone being so arranged that deflections of its ribbon due to incident sound waves causes changes in its inductance. The change in inductance is caused by air waves, which deflect the ribbon 50 Just as they do in the ordinary type of ribbon microphone in which minor currents are generated by making the ribbon cut a strong magnetic field supplied by a permanent magnet. Also,. any deflection of the ribbon will alter the 5 inductance of the system comprising the vertical trical characteristics are small and are suitably current energy at the said predetermined fre- 10 quency. For example, in the arrangement illustrated in Figs. 1 and 2 (which are mutually perpendicular views) the quarter wave length line consists of two concentric conductors I, 2,

each a quarter wave long, one being within the 15 other and held in spaced relation thereto by 'means of an insulating member 3. The inner conductor 2 is directly attached at one end to one end of the crimped conductive armature ribbon 4 of a ribbon microphone, the other end of said ribbon being attached to the cross member of a conductive U-shaped bridge piece 5 whose limbs contact directly with the adjacentend of the outer tubular conductor of the quarter wave line. In Fig.2, for the sake of clarity, the limbs 25 of the U are not shown. If desired, the space between the inner and outer conductors I, 2, of the line may for acoustic reasons he filled in with cotton wool orthe like and for the same reasons the interior construction at the end of the line 30 adjacent the ribbon armature may be of rounded form as shown by the dash line in Fig. 2. With this arrangement when sound strikes the ribbon armature inductance changes occur and these inductance changes directly affect the impedance 35 at the end of the quarter wave line. Since there are no leads between the microphone armature andthe line no swamping efiects" due to the lead self-inductances vor self-capacities will be experienced.

In the above described embodiment, wherein the microphone ribbon acts as a .variable inductance, themicrophone is a so-called velocity microphone. Of course, so-called pressure microphone arrangements could be employed. For example, a pressure inductance microphone armature might be constituted by a diaphragm supported on a thin collodion film or, as shown in Figs. 3 and 4, by a thin strip 6 of foil conductor wound round and supported on a thin collodion or similar film l. The ends of the foil strip 6 are electrically connected to the outside of conductor l and inner conductor 2, thus completing the electrical circuit.

The invention is not limited to the use of microphone members as acoustically variable inductances; for example, a condenser microphone could be employed with similar results at the end of a line a half wave length long.

Arrangements such as those above described and illustrated can be employed in circuits such as that illustrated in Fig. 5.

Referring to Fig. 5, a high frequency oscillator 8, for example an oscillator operating on a wave length of IS meters, is connected to apply an oscillatory input between grid 9 and cathode Ill of a tube H whose plate circuit includes an electrical line L of the nature described above and illustrated in Figs. 1, 2 and 3, terminated by a microphone member as above described. The plate circuit connections through line L will be obvious to those skilled in the art and needs no detailed description. Briefly, however, one member of line L say the inner conductor is connected by the usual blocking condenser as shownto the anode l2; this circuit being completed by connecting the other member say the outer conductor to the cathode l and to ground. Where the microphone member is such as to operate as an acoustically variable inductance, the'line should be a quarter of a wave length long or an odd multiple thereof to obtain the maximum voltage variation at the terminal end of the line. Where the microphone member is of the variable condenser type, the line should be a half wave length long or a multiple thereof to obtain the. maximum voltage variation at the terminal end of the line.

The anode 12 of the tube l I is capacity coupled by a condenser I3 to the grid Id of a further tube 15 and it will be appreciated that the input to this further tube will consist of the high frequency energy modulated by means of the microphone-line combination in accordance with incident sound upon the microphone. This modulated high frequency is taken from the anode circuit of the tube 15 and detected at IS the detected signals, of course, constituting the required audio frequency. Figure 5 is a highly simplified circuit diagram showing only those parts required for an understanding of this invention. Potential for the anode may be supplied through an inductance RFC although no anode potential sources are shown and only the line-microphone combination LM is shown in the anode circuit of the tube ll.

It will be appreciated that with arrangements in accordance with this invention, difficulties due to microphone leads may be practically completely avoided, while furthermore, relatively large outputs can be obtained for quite small sound inputs.

What is claimed is:

1. In an ultra high frequency modulating system a circuit in which ultra high frequency electrical wave energy to be modulated is flowing, a line comprising two conductors each connected 2. In an ultra high frequency signalling sys- I tem a source of ultra high frequency wave energy, a utilization circuit coupled thereto to be energized by wave energy therefrom, a source of modulating potentials and means for modulating the ultra high frequency wave energy energizing said utilizing means, comprising a transmission line including two concentric conductors each connected to said ultra high frequency source,.

a ribbon microphone, and means connecting the .ribbon of said microphone directly between points on said conductors, said microphone when line to effect modulation of said wave energy.

3. In a modulating system, a source of ultra high frequency wave energy, a utilization circuit, an electron discharge device having input electrodes coupled to said source of ultra high frequency wave energy and output electrodes coupled to said utilization circuit, a line the length of which is an integral number of quarter wave lengths of the wave energy of said source, said line comprising two concentric conductors the outer conductor of which is tubular, means connecting adjacent terminals of said conductors between electrodes in said discharge device and means connecting a ribbon microphone between the other ends of said conductors.

4. In a modulation system circuit wherein ultra high frequency Wave energy to be modulated is flowing, a line comprising an inner conductor surrounded by and insulated from an outer tubular conductor, a ribbon microphone connected directly between one end of said inner conductor and the adjacent end of said outer conductor, and means connecting the free end of said inner conductor and said outer conductor to said ultra high frequency circuit, said inner conductor being an odd number of quarter wave lengths of the ultra high frequency wave energy flowing in said circuit whereby changes in inductance of the ribbon will cause corresponding changes in impedance between the end of the said inner conductor connected to said ultra high frequency circuit and the outer conductor to thereby modulate the ultra high frequency wave energy flowing in said circuit.

NOEL MEYER RUST.

25 subjected to sound waves acting through said 

